Organic thin film transistors are expected to be utilized in driving circuits for organic light-emitting diodes (OLEDs), radio-frequency ID tags, sensors, and the like, because a low-temperature deposition process is applicable to them and they can be easily deposited on a flexible substrate and the like. Performance of a device depends on the interface between an electrode and an organic semiconductor. That is, it is considered that electrical contact resistance exists between an organic semiconductor layer and an electrode and that the energy difference between the work function of the highest occupied molecular orbital (HOMO) of the organic semiconductor and that of the electrode has a large effect. Gold electrodes, which generally have a work function around 5.1 eV, match well with the HOMO of p-type organic semiconductors and are frequently used. However, research using silver and copper are also underway because of cost issues. Silver electrodes, which have a work function of 4.26 eV and copper electrodes, which have a work function of 4.6 eV, do not match well with organic semiconductors. As a measure to solve this problem, attempts have been made to perform surface treatment on a source-drain electrode to thereby change its work function and to lower the electrical charge injection barrier between the electrode and an organic semiconductor layer.
For example, NPL 1 discloses that an attempt has been made to perform surface treatment of a metal electrode with pentafluorothiophenol to thereby form a self-assembled monolayer (SAM) on the electrode and to change the work function of the electrode surface. Thiols have such features that they form strong bonds on metals and thus have high durability and that a substituent having a high electronegativity such as fluorine increases their work function. Such organic thin film transistors (TFTs) have good characteristics, and solution processes with a high production efficiency can be applicable to the TFTs.
Alternatively, NPL 2 discloses that two-component SAMs of C8F17C2H4SH and C10H21SH are used, and the work function can be desirably regulated by changing the ratio between the two components. Surface modification by use of such two-component SAMs has achieved superior performance to that of organic TFTs including a gold electrode.
NPL 3 discloses that thioacetic acid, which is used for thiol synthesis and tends to be mixed as an impurity, has been intentionally added to thiol to form SAMs, and the influence of the impurity on the SAMs has been investigated. It is indicated that a trace amount of an impurity mixed in thiol causes competitive absorption onto gold and defects are included in the monolayer. In this case, the greater the amount of thioacetic acid, the more defects there are in the monolayer.
Additionally, reactive silanes such as silane coupling agents, as surface treatment agents, can form SAMs on oxide materials such as silicone oxides, titanium oxides, ITOs, aluminum oxides, glass, tin oxides, and germanium oxides, and also on metal materials such as silicon, titanium, and aluminum, via their surface oxide films.
NPL 4 discloses a method for changing the work function by depositing a self-assembled monolayer (F-SAM) of heptadecafluorodecyl triethoxysilane, which is a reactive silane, on an ITO.